1. Field of the Invention
The present invention relates generally to an optical fiber drawing device, and in particular, to an optical fiber coating device with a cooler component.
2. Description of the Related Art
In general, all fibers share the same fundamental structure. The center of the fiber is the core, which has a higher refractive index than the cladding that surrounds the core. The difference in the refractive index causes total internal reflection that guides light along the core. The size of core and cladding can vary depending on the type of images or light for illumination. A plastic coating increases the diameter of the fibers. The goal of the coating is to provide easier handling and to protect the fiber surfaces from scratches and other mechanical damages.
Most fibers are made of pure glass, such as a sillica-based glass, with small levels of impurities to adjust the refractive index. Various amounts of information including video images and computer data are converted into optical signals, then propagated along the optical fiber. Due to inherent glass characteristics, the optical fiber is vulnerable to external tension, sharp bending forces, and various stresses. In particular, the optical fiber is acutely vulnerable to tension applied in the elevation direction.
Mechanically, glass fibers are stiff but flexible and strong. In order to test the optical properties, a destructive tension test is often performed to measure the tension-resistance of the optical fiber. The destructive tension test is characterized by longitudinally pulling the optical fiber and measuring the extent of the whole fiber length by which it is stretched before inducing mechanical damage along the fiber surfaces. A typical optical fiber is known to be stretched approximately 0.5% more than its original length before mechanical damage. Accordingly, plastic coating may not affect the propagation characteristics of the electric wave along the fiber, but may be regarded as an essential element in protecting the optical fiber surfaces from mechanical damage.
The material of the coating is typically selected from the group consisting of different plastics, i.e., polyester materials, ultraviolet-hardened resins, thermal-hardened resins, etc. Bare glass fibers are coated with plastic through an extrusion technique as they are manufactured to protect from physical damage. Here, the bare glass refers to an uncoated optical fiber, namely, an optical fiber composed of only a core and a clad. Then, the plastic materials that are applied to the optical fiber through the extrusion manner are hardened by a cooler, such as a cooling reservoir filled with cooling water. Alternatively, the bare glass fibers are coated with ultraviolet-hardened resins in liquid form; then, ultraviolet rays are irradiated to the optical fiber for hardening purposes. In another alternative, the bare glass fibers are coated with thermal-hardened resins in liquid form and then hardened by the heat treatment.
An optical drawing device is used to coat the bare glass fibers with the ultraviolet-hardened resin, and the device typically includes a furnace, a coating device, an ultraviolet hardener, a capstan, and a spool. The furnace has a cylindrical shape and applies heat to one end of an optical fiber preform, which consists of a core and clad member, in order to melt the optical fiber preform. Here, the optical fiber preform is similar to the bare glass in terms of their composition, but the diameter is much greater than that of the bare glass. Also, during the heat application process, an inert gas flows into the furnace to prevent any combustion inside the furnace during the heat treatment.
FIG. 1 is a simplified diagram illustrating the construction of a coating device according to a prior art. Basically, this coating device includes a coater 12 with a first and second applicators 13 and 14, and annex devices including a gas provider 19, a gas controller 18, a filter 17, a coating material reservoir (not shown), etc. The gas controller 18 controls the amount of gas supplied to the coater 12 via the filter 19. The coater 12 includes a chamber for mounting the first and second applicators 13 and 14. A passageway is formed inside the coater 12, so that the bare glass 11 can pass through the passage way. The passage way also receives the gas provided from the gas provider 19 to eliminate bubbles formed along the fiber during the coating process. The first and second applicators 13 and 14 are filled with the coating materials provided from the coating material reservoir in order to actuate the coating along the passing bare glass 11. Hence, the bare glass 11 is coated twice while passing through the first and second applicators 13 and 14. Then, the ultraviolet hardener is positioned at the exit of the coating device to harden the optical fiber by irradiating ultraviolet rays into the coating of the optical fiber.
In addition, the first and second applicators 13 and 14 are configured to execute a double coating to reduce physical damages associated with cabling, installation, or environmental changes during the service life of the optical fiber. The coating material provided in the first applicator 13 includes a softer physical property than the materials provided in the second applicator 14. However, as the optical fiber is passed through the coating materials, bubbles may be entrapped along the fiber. If the kinematic viscosity of gas provided from the gas provider 19 is large, the gas is entered into the coating material easily and gets adhered to the optical fiber. A part of this entered gas is dissolved but other part remains in the form of bubbles. Thus, the entrapment of bubbles is determined by the kinematic viscosity of gas inside the chamber as well as the surface characteristics of the optical fiber. Also, the pressure fluctuation inside the coating chamber during the coating process affects the amount of gas entrapped along the surface of the optical fiber.
A capstan-type device is used to draw the optical fiber with a predetermined power, so as to consecutively draw the optical fiber from the optical fiber preform at specific intervals. The spool having a cylindrical reel shape causes the drawn optical fiber wind around outer circumference thereof. The entrapment of bubbles, as described in the preceding paragraphs, becomes problem when the drawing speed is increased. Yet, in order to increase the productivity, optical fibers are frequently pulled at a faster rate, which in turn lower the required kinematic viscosity of the environmental gas provided in the coating device. That is, if the drawing velocity is increased, the amount of bubbles produced within the coater 12 and the probability of gas trapped in the coating material is increased since the contact angle of a core-cladding layer to the coating material becomes smaller. Accordingly, it requires extra time for the entrapped gas to be dissolved in the coating material. Moreover, the coating material is exposed to the UV-ray before the entrapped gas is completely dissolved, thereby being hardened and remained in the form of bubbles.
FIG. 2 is a diagram illustrating bubbles remaining behind during the coating process of an optical fiber at a higher drawing speed. The bubbles 25 generated, as explained in the preceding paragraphs, remain in the first coated coating 23 surrounding the outer circumference of the bare glass 22 of an optical fiber 21. Referring to FIG. 2, the outward appearance of a second coated coating 24 is preserved, but with heavy remnants the outer circumference of the optical fiber 21 would be bulged. Or, if the bubbles formed in the coater 12 through the device shown in FIG. 1 burst when passing the ultraviolet hardener positioned at the bottom of the coater 12, the outer circumference of the optical fiber 21 would be dented.
Such defective optical fiber can not be used and requires the defective areas to be eliminated later. A metallic electric wire can be easily replaced but such operation is impractical as the fiber is very susceptible to any external impact, shearing force, scratching, tension, etc.
To prevent the above problems, the environmental gas with a quite lower kinematic viscosity has been proposed to be utilized when the optical fiber drawing velocity becomes faster. Some of the environmental gas having a low kinematic viscosity includes, for example, CCI2F2, Xe, etc. However, the proposed gases have other drawbacks as they tend to be expensive, require a delicate storage means, and are harmful to handle.